The refractive index profile of an optical fiber is generally descried in terms of the appearance of a graph plotting the refractive index of the fiber as a function of radius. In conventional manner, the distance r to the center of the fiber is plotted along the abscissa axis, and the difference between the refractive index at that distance and the refractive index of the fiber cladding is plotted up the ordinate axis. Index profiles are thus described as being “step-shaped”, “trapezium-shaped”, or “triangular” for graphs that are respectively step-shaped, trapezium-shaped, or triangular in appearance. These curves generally represent a theoretical or reference profile for a fiber, and fiber manufacturing constraints can lead to a profile that departs perceptibly therefrom. Index variation along the profile is used to control light propagation along the fiber.
So-called “photonic” or “photonic crystal” fibers (PCF) have recently appeared: unlike conventional fibers, these fibers are not entirely constituted by a transparent solid material such as doped silica; in section, a photonic fiber presents a plurality of air holes. These holes are parallel to the axis of the fiber, and they extend longitudinally along the fiber. In practice, the holes can be obtained by making the preform by assembling capillary tubes and cylinders of silica to build up the pattern of holes to be obtained in the fiber. Drawing down such a preform provides a fiber with holes corresponding to the capillary tubes.
The presence of these holes in the fiber material gives rise to variations in the mean index of the material; as in a conventional optical fiber, these index variations can be used for guiding light signals at appropriate wavelengths. A description of such photonic fibers is to be found in WO-A-00 49 435: in addition to describing the principle on which photonic fibers operate, that document also describes a method enabling such fibers to be assembled, with hole diameter varying longitudinally. The index profile of the fibers is not specified; the application mentions that the variations in mode diameter caused by the longitudinal variations in the sizes of the holes can be used in optical amplifiers.
R. F. Cregan et al. in “Distribution of Spontaneous Emission from an Er3-doped Photonic Crystal Fiber”, Journal of Lightwave Technology, Vol. 17, No. 11, November 1999, investigates spontaneous emission in a photonic fiber. The air holes are distributed in a triangular matrix, the fiber being hexagonal in shape; in the center of the hexagon, the fiber does not present a hole, and the silica is doped with erbium. That document studies the three-dimensional distribution of spontaneous emission while the fiber is being pumped axially; it shows that the distribution is a function of how the holes are distributed in the fiber, in agreement with simulation. No reference is made to any use for the doped fiber.
Thomas Sondergaard, in “Photonic Crystal Distributed Feedback Fiber Lasers with Bragg Gratings”, Journal of Lightwave Technology, Vol. 18 No. 4, April 2000, discusses the use of photonic fibers for making fiber lasers; he states that the mode areas for the signal or for the pump can be smaller than or greater than the corresponding mode areas in conventional step index fibers. The use of photonic fibers thus makes it possible to make lasers having a low pumping threshold (for small mode areas), or to make high power lasers (for large mode areas). That document mentions digital simulations only, and does not describe any practical embodiments.
W. J. Wadsworth et al. in “Yb3+-doped Photonic Crystal Fiber Laser”, Electronics Letters, Vol. 36 NO. 17, August 2000, demonstrates a laser effect experimentally in a photonic fiber; the fiber is made taking a silica tube doped with Yb and codoped with Al and surrounding it with capillaries of pure silica; the assembly is then drawn down to form a fiber, and a sleeve of pure silica is placed around it. Two rows of holes surround the doped core, and light is strongly confined in the doped core of the fiber.
EP-A-1 043 816 describes a double-clad fiber; the signal travels in the doped core of the fiber, and a pump is injected into the first cladding; the effect of the second cladding is to confine the pumping light within the first cladding. In order to direct the pumping light towards the doped core, proposals are made to provide regions of modified index in the first cladding. Those regions of modified cladding can be constituted in particular by air holes. In one embodiment, three modified index regions are provided that are distributed around the periphery of the first cladding. In another embodiment, six modified index regions are provided at the vertices and at the middles of the sides of an equilateral triangle. It is suggested that the modified index regions should be disposed as far away as possible from the core of the fiber in order to avoid modifying polarization within the core of the fiber.